Molten metals, particularly molten aluminum and steel, are frequently contaminated by entrained non-metallic inclusions that give rise to a variety of shortcomings or defects in resulting products. These inclusions may cause the metal to tear during mechanical working operations, pin-holes and streaks in foils, surface defects and blisters in sheets, and increased rates of breakage during the production of wire.
One analyzer that provides quick results and includes size and concentration information of inclusions is a Liquid Metal Cleanliness Analyzers (LiMCA). An LiMCA may comprise an electrically insulating wall means, often in the form of a sampling tube, having a small, precisely-dimensioned passage in a side wall. The tube is immersed in the molten metal to be tested and a uniform stream thereof is passed through the passage while a substantially constant electric current is established through the stream between two electrodes disposed respectively inside and outside the tube. The particles of interest have very high resistivity compared to the molten metal and travel of a particle through the passage is accompanied by a change in resistance against the electric current producing an electrical pulse in the voltage. The number of pulses produced while a fixed volume of metal passes through the passage provides an indication of the number of particles per unit volume in the metal. Furthermore it is possible to analyze the pulse shape to determine particle size and size distribution.
The LiMCA apparatus has been designed for “on-line” tests, to give results in seconds, but this often means that the apparatus is close to the molten metal source and associated noise generating equipment.
Within the industrial environment of the LiMCA, there may be many sources of electrical or mechanical interference, or noise that affect the results of electrical pulse detection. It is difficult in practice to reliably extract the wanted pulse signals of the LiMCA system from these noise signals, since the noise may be of the same order of magnitude as the wanted pulse signals from detection of the smaller particles. To this end, it has traditionally been preferred that the supply current come from rechargeable batteries. Such batteries, for example Nickel-Cadmium types, can be recharged at points during the measurement cycle when data is not actually being collected but the use of batteries requires additional steps and equipment to ensure that charging is properly controlled. The batteries are also sensitive to high temperatures that can occur in the environment and are limited in the number of charge-discharge cycles that can be applied. A vacuum or pressure source is generally used to move the metal through the passage and should be free of pump-generated pulses that may interfere with the signal. The entire apparatus should also be shielded as much as possible against outside electromagnetic interference.
The design and use of filters to reduce or eliminate interference is now a well-developed art, but difficulties arise when used with LiMCA analyzers, due to the relatively low voltage signal characteristic of the particle-indicating pulses and the fact that the pulse frequencies, corresponding to the number of particles per unit time passing through the passage, are of the same order of magnitude as those of many of the interfering noise pulses. Shielding can be provided to reflect or absorb the broadcast radiation before it reaches the apparatus but it is impossible to achieve a perfect shielding because of the need for inputs and outputs to and from the system.
Typical LiMCA analyzers are described in U.S. Pat. Nos. 4,600,880, 5,130,639, 4,555,662 and 5,039,935, herein incorporated by reference. U.S. Pat. No. 5,130,639 (Hachey) describes the use of various combinations of electrodes in a LiMCA analyser where the measurement and current supply electrodes are separate and placed in noise reducing configurations. At least four, and up to six electrodes are required.
U.S. Patent Application No. 2002/0067143, also herein incorporated by reference, describes the use of ultra-capacitors as alternatives for batteries in certain applications. Ultra-capacitors can be used as power sources in harsh environments because they are less temperature sensitive than batteries and they can be rapidly charged and discharged compared to batteries, thereby requiring less control on the charging-discharging process, and have very large cycle life compared to batteries. However, ultra-capacitors have a lower volume charge density than rechargeable batteries and cannot therefore supply constant high currents for extended periods of time compared to batteries.
It is generally seen as desirable to develop an analyzer system that can reduce interference while simplifying design and requiring fewer additional parts.
It is further desirable to develop an analyzer system that can operate with increased continuity and overcome the deficiencies of battery operation.